Verilog Programming part 23

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Examples In order to illustrate the use of behavioral constructs discussed earlier in this chapter, we consider three examples in this section. The first two, 4-to-1 multiplexer and 4-bit counter, are taken from Section

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Nội dung Text: Verilog Programming part 23

7.9 Examples In order to illustrate the use of behavioral constructs discussed earlier in this chapter, we consider three examples in this section. The first two, 4-to-1 multiplexer and 4-bit counter, are taken from Section 6.5, Examples. Earlier, these circuits were designed by using dataflow statements. We will model these circuits with behavioral statements. The third example is a new example. We will design a traffic signal controller, using behavioral constructs, and simulate it. 7.9.1 4-to-1 Multiplexer We can define a 4-to-1 multiplexer with the behavioral case statement. This multiplexer was defined, in Section 6.5.1, 4-to-1 Multiplexer, by dataflow statements. It is described in Example 7-35 by behavioral constructs. The behavioral multiplexer can be substituted for the dataflow multiplexer; the simulation results will be identical. Example 7-35 Behavioral 4-to-1 Multiplexer // 4-to-1 multiplexer. Port list is taken exactly from // the I/O diagram. module mux4_to_1 (out, i0, i1, i2, i3, s1, s0); // Port declarations from the I/O diagram output out; input i0, i1, i2, i3; input s1, s0; //output declared as register reg out; //recompute the signal out if any input signal changes. //All input signals that cause a recomputation of out to //occur must go into the always @(...) sensitivity list. always @(s1 or s0 or i0 or i1 or i2 or i3) begin case ({s1, s0}) 2'b00: out = i0; 2'b01: out = i1; 2'b10: out = i2;
2'b11: out = i3; default: out = 1'bx; endcase end endmodule 7.9.2 4-bit Counter In Section 6.5.3, Ripple Counter, we designed a 4-bit ripple carry counter. We will now design the 4-bit counter by using behavioral statements. At dataflow or gate level, the counter might be designed in hardware as ripple carry, synchronous counter, etc. But, at a behavioral level, we work at a very high level of abstraction and do not care about the underlying hardware implementation. We will design only functionality. The counter can be designed by using behavioral constructs, as shown in Example 7-36. Notice how concise the behavioral counter description is compared to its dataflow counterpart. If we substitute the counter in place of the dataflow counter, the simulation results will be exactly the same, assuming that there are no x and z values on the inputs. Example 7-36 Behavioral 4-bit Counter Description //4-bit Binary counter module counter(Q , clock, clear); // I/O ports output [3:0] Q; input clock, clear; //output defined as register reg [3:0] Q; always @( posedge clear or negedge clock) begin if (clear) Q
endmodule 7.9.3 Traffic Signal Controller This example is fresh and has not been discussed before in the book. We will design a traffic signal controller, using a finite state machine approach. Specification Consider a controller for traffic at the intersection of a main highway and a country road. The following specifications must be considered: • The traffic signal for the main highway gets highest priority because cars are continuously present on the main highway. Thus, the main highway signal remains green by default. • Occasionally, cars from the country road arrive at the traffic signal. The traffic signal for the country road must turn green only long enough to let the cars on the country road go. • As soon as there are no cars on the country road, the country road traffic signal turns yellow and then red and the traffic signal on the main highway turns green again. • There is a sensor to detect cars waiting on the country road. The sensor sends a signal X as input to the controller. X = 1 if there are cars on the country road; otherwise, X= 0. • There are delays on transitions from S1 to S2, from S2 to S3, and from S4 to S0. The delays must be controllable. The state machine diagram and the state definitions for the traffic signal controller are shown in Figure 7-1. Figure 7-1. FSM for Traffic Signal Controller Verilog description The traffic signal controller module can be designed with behavioral Verilog constructs, as shown in Example 7-37.
Example 7-37 Traffic Signal Controller `define TRUE 1'b1 `define FALSE 1'b0 //Delays `define Y2RDELAY 3 //Yellow to red delay `define R2GDELAY 2 //Red to green delay module sig_control (hwy, cntry, X, clock, clear); //I/O ports output [1:0] hwy, cntry; //2-bit output for 3 states of signal //GREEN, YELLOW, RED; reg [1:0] hwy, cntry; //declared output signals are registers input X; //if TRUE, indicates that there is car on //the country road, otherwise FALSE input clock, clear; parameter RED = 2'd0, YELLOW = 2'd1, GREEN = 2'd2; //State definition HWY CNTRY parameter S0 = 3'd0, //GREEN RED S1 = 3'd1, //YELLOW RED S2 = 3'd2, //RED RED S3 = 3'd3, //RED GREEN S4 = 3'd4; //RED YELLOW //Internal state variables reg [2:0] state; reg [2:0] next_state;
//state changes only at positive edge of clock always @(posedge clock) if (clear) state
S2: begin //delay some positive edges of clock repeat(`R2GDELAY) @(posedge clock); next_state = S3; end S3: if(X) next_state = S3; else next_state = S4; S4: begin //delay some positive edges of clock repeat(`Y2RDELAY) @(posedge clock) ; next_state = S0; end default: next_state = S0; endcase end endmodule Stimulus Stimulus can be applied to check if the traffic signal transitions correctly when cars arrive on the country road. The stimulus file in Example 7-38 instantiates the traffic signal controller and checks all possible states of the controller. Example 7-38 Stimulus for Traffic Signal Controller //Stimulus Module module stimulus; wire [1:0] MAIN_SIG, CNTRY_SIG; reg CAR_ON_CNTRY_RD; //if TRUE, indicates that there is car on //the country road reg CLOCK, CLEAR; //Instantiate signal controller sig_control SC(MAIN_SIG, CNTRY_SIG, CAR_ON_CNTRY_RD, CLOCK, CLEAR); //Set up monitor initial
$monitor($time, " Main Sig = %b Country Sig = %b Car_on_cntry = %b", MAIN_SIG, CNTRY_SIG, CAR_ON_CNTRY_RD); //Set up clock initial begin CLOCK = `FALSE; forever #5 CLOCK = ~CLOCK; end //control clear signal initial begin CLEAR = `TRUE; repeat (5) @(negedge CLOCK); CLEAR = `FALSE; end //apply stimulus initial begin CAR_ON_CNTRY_RD = `FALSE; repeat(20)@(negedge CLOCK); CAR_ON_CNTRY_RD = `TRUE; repeat(10)@(negedge CLOCK); CAR_ON_CNTRY_RD = `FALSE; repeat(20)@(negedge CLOCK); CAR_ON_CNTRY_RD = `TRUE; repeat(10)@(negedge CLOCK); CAR_ON_CNTRY_RD = `FALSE; repeat(20)@(negedge CLOCK); CAR_ON_CNTRY_RD = `TRUE; repeat(10)@(negedge CLOCK); CAR_ON_CNTRY_RD = `FALSE; repeat(10)@(negedge CLOCK); $stop; end endmodule Note that we designed only the behavior of the controller without worrying about how it will be implemented in hardware.